Electro-optic modulator
By introducing a nipn junction structure and using III-V semiconductor materials in the MOSCAP modulator, the problems of high optical loss and low efficiency in the modulator are solved, achieving high bandwidth and high efficiency electro-optic modulation effect.
Patent Information
- Application Number
- CN202080066681.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Priority Date
- 2019-11-21
- Filing Date
- 2020-07-23
- Publication Date
- 2025-12-05
- Estimated Expiration
- 2040-07-23
AI Technical Summary
Existing modulators based on metal-oxide-semiconductor capacitors (MOSCAPs) suffer from high optical loss and low modulation efficiency due to low hole mobility and large capacitance.
The nipn junction structure is adopted, and a second n-doped region is introduced in the modulation region to reduce the series resistance, while the p region is retained to provide high modulation efficiency. III-V semiconductors such as indium phosphide are used to improve carrier mobility.
An electro-optic modulator with high bandwidth and high modulation efficiency was achieved, with a 50% increase in bandwidth, improved carrier conductivity, and faster response time, while maintaining similar electric field strength and charge accumulation.
Smart Images
Figure CN114503020B_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present invention relates to an electro-optic modulator. BACKGROUND
[0002] Metal oxide semiconductor capacitor (MOSCAP) based modulators typically have a large capacitance due to the thin dielectric layer forming the capacitor region. The larger capacitance slows down the modulator as a large amount of charge has to be dissipated.
[0003] The modulation efficiency increases with thinner dielectric layers, however this comes at the cost of increased capacitance. Therefore, to achieve high bandwidth, the series resistance of the modulator must be as small as possible.
[0004] In examples of known MOSCAP modulators, a p-i-n junction is formed, in which: a lower doped (n or p) region is vertically separated from an upper doped (p or n) region by a laterally extending insulating layer; or, a left doped (n or p) region is laterally separated from a right doped (p or n) region by a vertically extending insulating layer.
[0005] However, the hole mobility of semiconductors that can be used for silicon photonics applications is an order of magnitude lower than silicon. This lower hole mobility results in higher resistance, and therefore higher optical loss for the same doping density. This means that the p-side of a MOSCAP device limits the overall performance. If an n-i-n junction is provided, the modulation efficiency is lower due to the lack of carrier accumulation and depletion at the interfaces. SUMMARY
[0006] In a first aspect, embodiments of the present invention provide an electro-optic modulator based on a metal oxide semiconductor capacitor, MOSCAP, comprising:
[0007] an input waveguide;
[0008] a modulation region coupled to the input waveguide; and
[0009] an output waveguide coupled to the modulation region;
[0010] wherein the modulation region comprises an n-i-p-n junction, the n-i-p-n junction comprising:
[0011] a first n-doped region spaced apart from a p-doped region by an intrinsic region; and a second n-doped region spaced apart from the intrinsic region by the p-doped region and on an opposite side of the intrinsic region to the first n-doped region.
[0012] The retention of the p-region results in high modulation efficiency, and the second n-doped region reduces the series resistance.
[0013] A MOSCAP modulator may have any of the following optional features or any combination of the following optional features to a degree of compatibility with the following optional features.
[0014] The n-doped region can be doped with any of the following: phosphorus, arsenic, antimony, bismuth, and lithium. The p-doped region can be doped with any of the following: boron, aluminum, gallium, and indium.
[0015] The p-doped region can be thinner than either or both of the first n-doped region or the second n-doped region. The thickness of the p-doped region can be equal to the thickness of the intrinsic region. By providing a p-doped region of such size, a high field is provided for carrier modulation. A wider n-doped region provides a lower access resistance.
[0016] The p-doped region can be less than 200 nm thick. The p-doped region can be less than 100 nm thick.
[0017] Intrinsic regions can be formed by oxides.
[0018] The MOSCAP modulator may also include a first electrode connected to a first n-doped region and a second electrode connected to a second n-doped region.
[0019] The intrinsic region can extend obliquely through the modulation region.
[0020] A nipn junction can be a vertical junction, where the first n-doped region is the bottom layer and the second n-doped region is the top layer.
[0021] The nipn junction can be a horizontal junction, wherein the first n-doped region is on the first lateral side of the modulator, and the second n-doped region is on the second lateral side of the modulator.
[0022] The modulator can have an operating bandwidth in the range of 30 GHz to 40 GHz.
[0023] The first n-doped region, the second n-doped region, and the p-doped region can be formed from the same semiconductor material.
[0024] The first n-doped region can be formed of a semiconductor material that is different from the second n-doped region and the p-doped region.
[0025] At least one of the first n-doped region, the second n-doped region, and the p-doped region may be formed of a III-V semiconductor. The III-V semiconductor may be indium phosphide.
[0026] In a second aspect, embodiments of the present invention provide a method for manufacturing a MOSCAP modulator, the method comprising: on a substrate:
[0027] A first semiconductor region is grown and doped with an n-type dopant to form a first n-doped region;
[0028] An insulator is grown on the first surface of the first n-doped region;
[0029] A second semiconductor region is grown on the second surface of the insulator, with the first surface opposite to the second surface;
[0030] A first portion of the second semiconductor region is doped with a p-type dopant to form a p-doped region adjacent to the insulator; and
[0031] The second portion of the second semiconductor region is doped with an n-type dopant to form an n-doped region adjacent to the p-doped region.
[0032] The method may have any or any combination of the optional features of the first aspect, as long as they are compatible.
[0033] In a third aspect, embodiments of the present invention provide a MOSCAP modulator manufactured according to the second aspect. Attached Figure Description
[0034] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:
[0035] Figure 1 A MOSCAP modulator including a vertical nipn junction is shown;
[0036] Figure 2 A MOSCAP modulator including a horizontal nipn junction is shown;
[0037] Figure 3 A MOSCAP modulator including a tilted nipn junction is shown;
[0038] Figure 4 It is a graph showing the bandwidth difference between a nip junction and a nipn junction;
[0039] Figure 5A and Figure 5B These are the band structure curves of a nip junction and a nipn junction, respectively; and
[0040] Figure 6A and Figure 6B These are curves showing charge accumulation in a nip junction and a nipn junction, respectively. Detailed Implementation
[0041] Various aspects and embodiments of the invention will now be discussed with reference to the accompanying drawings. Other aspects and embodiments will be apparent to those skilled in the art.
[0042] Figure 1 A MOSCAP modulator 100 including a vertical nipn junction is shown. The junction includes a first n-doped region 101, which in this example is a horizontally extending semiconductor layer (i.e., in line with a substrate not shown). The first n-doped region 101 is vertically spaced from a second n-doped region 102 by an insulator 103 and a p-doped region 104. The insulator is an oxide, such as silicon dioxide, and the n-doped and p-doped regions may be formed of silicon or silicon-germanium.
[0043] according to Figure 1 The modulator is typically fabricated on a wafer. A first semiconductor layer is doped and then etched to provide a first n-doped region 101. A cladding or insulating material is grown to the right of the n-doped region 101, i.e., below the region providing the second n-doped region 102. Next, an oxide layer is formed on top of both the first n-doped region and the cladding or insulating material, and the left and side edges are etched back to define an insulator 103. Another semiconductor layer is grown or deposited and then doped with n-type and p-type dopants. The p-doped region can be formed, for example, by deep implantation of the dopant. The other semiconductor layer is etched back to define the second n-doped region 104 and the p-doped region 103.
[0044] Figure 2 A MOSCAP modulator including a horizontal nipn junction is shown. The junction includes a first n-doped region 202, which in this example is a semiconductor layer that extends horizontally and also has vertically extending segments (extending away from the substrate). The first n-doped region 202 is horizontally spaced from a second n-doped region 202 by an insulator 203 and a p-doped region 204. Again, the insulator is an oxide, such as silicon dioxide, and the p-doped and n-doped regions can be formed of silicon or silicon-germanium. Figure 2 In the variant (not shown), the first n-doped region, the second n-doped region, the p-doped region, and the insulator all have the same height (i.e., extend vertically).
[0045] according to Figure 2 The modulator is also typically fabricated on a wafer. A first semiconductor layer is etched away to provide the geometry of a first n-doped region 201, and then doped to provide the first n-doped region 201. Next, an insulating layer 203 is provided via oxidation, deposition, or other methods. Subsequently, another conductor is deposited and optionally etched to provide the geometry of a p-doped region 204 and a second n-doped region 202. Then, p-dopants and n-dopants are deposited to provide the p-doped region 204 and the second n-doped region 202.
[0046] Figure 3A MOSCAP modulator including a tilted nipn junction is shown. The junction includes a first n-doped region 301, which in this example is a semiconductor layer that extends horizontally and also has a vertically extending segment (extending away from the substrate). The first n-doped region 301 is horizontally and vertically spaced from a second n-doped region 302 by an insulator 303 and a p-doped region 304. The interface between the insulator 303 and the p-doped region 304 is tilted, extending in both vertical and horizontal directions. Similarly, the interface between the insulator 303 and the first n-doped region 301 is also tilted. In this example, the interface between the p-doped region 304 and the second n-doped region 302 is not tilted, extending entirely vertically. However, the interface between the p-doped region 304 and the second n-doped region can be tilted, and can have the same angle as the interface between the insulator 303 and the first n-doped region 301.
[0047] according to Figure 3 The modulator can be used with respect to... Figure 2 The method discussed is similar to that used in this case. However, in this case, selective etching will be used to create the tilted interface between the first n-doped region 301 and the insulator 303. Such selective etching can be performed using etching techniques that have the property of etching along preferred crystal planes. After this selective etching, the insulator 303, the p-doped region 304, and the second n-doped region 302 can be produced as discussed above. In the example where the interface between the p-doped region 304 and the second n-doped region 302 is tilted, the tilted interface can be created by implanting p-type dopant at an angle other than 90°.
[0048] Figures 1 to 3 The modulator shown is located within a waveguide. In some examples, the waveguide is a ridge waveguide, where the optical mode is primarily confined to the upper ridge portion of the waveguide (rather than the lower plate-like portion). In other examples, the waveguide is a rib waveguide, where the optical mode is primarily confined to the plate-like portion and guided by the upper rib portion.
[0049] exist Figures 1 to 3 In the modulator shown, the first n-doped region, the second n-doped region, and the p-doped region can be formed of the same semiconductor material (e.g., silicon, silicon-germanium, III-V semiconductor, indium phosphide, etc.). Alternatively, the first n-doped region and the second n-doped region can be formed of different semiconductor materials. For example, the first n-doped region can be formed of silicon or silicon-germanium, and the second n-doped region can be formed of indium phosphide or another III-V semiconductor. The p-doped region is typically formed of the same semiconductor material as the second n-doped region, but it can also be formed of a different semiconductor material.
[0050] Figure 4This is a graph showing the bandwidth difference between a nip junction and a nipn junction. It can be seen that for the same oxide thickness (and therefore capacitance), moving from a nip junction to a nipn junction results in a 50% increase in bandwidth.
[0051] Figure 5A and Figure 5B These are band structure graphs for a nip junction and a nipn junction, respectively. A graph is a curve showing energy (y-axis, e.g., electron volts) versus position (z, measured in micrometers) within the modulator. The lines represent different bands: Ec – conduction band; Ev – valence band; Ei – intrinsic Fermi level; Efn – electron Fermi level; and Efp – hole Fermi level.
[0052] It is noteworthy that the slope gradient near 0 micrometers (i.e., within the junction) determines the electric field strength of the modulator, which affects efficiency. As is known, this electric field is generated by the juxtaposition of n-doped and p-doped regions. It can be observed that the electric field strength at the junction for a nip junction is similar to that for a nipn junction, both exhibiting similar energy variations.
[0053] Advantageously, the second n-doped region then provides better conductivity and therefore faster response time than the nip junction, while maintaining a similar level of field strength and therefore efficiency.
[0054] Figure 6A and Figure 6B These are graphs showing charge accumulation in a nip junction and a nipn junction, respectively. The y-axis of the graphs shows the number of charge carriers, and the x-axis shows the location in the modulator (0 at the junction). It can be seen that despite the addition of another n-doped region, a similar number of charge carriers exist at the interface of the nipn junction as in the nip junction.
[0055] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when this disclosure is given. Therefore, the exemplary embodiments of the invention set forth above are to be considered illustrative rather than restrictive. Various changes may be made to the embodiments without departing from the spirit and scope of the invention.
Claims
1. A metal-oxide-semiconductor capacitor (MOSCAP) based electro-optic modulator comprising: an input waveguide; a modulation region coupled to the input waveguide; and an output waveguide coupled to the modulation region; wherein the modulation region comprises an n-i-p-n junction comprising: a first n-doped region spaced apart from a p-doped region by an insulator region; and a second n-doped region spaced apart from the p-doped region by the insulator region and on an opposite side of the insulator region to the first n-doped region, wherein the insulator region extends through the modulation region at an oblique angle, such that an interface between the insulator region and the first n-doped region is oblique, and an interface between the insulator region and the p-doped region is oblique, and wherein the first n-doped region is spaced apart from the second n-doped region in both horizontal and vertical directions by the insulator region and the p-doped region.
2. The MOSCAP modulator of claim 1, wherein the p-doped region is thinner than either or both of the first n-doped region or the second n-doped region.
3. The MOSCAP modulator of claim 1 or claim 2, wherein a thickness of the p-doped region is equal to a thickness of the insulator region.
4. The MOSCAP modulator of any preceding claim, wherein a thickness of the p-doped region is less than 200 nm.
5. The MOSCAP modulator of any preceding claim, wherein a thickness of the p-doped region is less than 100 nm.
6. The MOSCAP modulator of any preceding claim, wherein the insulator region is formed from an oxide.
7. The MOSCAP modulator of any preceding claim, further comprising a first electrode connected to the first n-doped region and a second electrode connected to the second n-doped region.
8. The MOSCAP modulator of any of claims 1 to 6, wherein the n-i-p-n junction is a vertical junction such that the first n-doped region is a lowermost layer and the second n-doped region is an uppermost layer.
9. The MOSCAP modulator of any of claims 1 to 6, wherein the n-i-p-n junction is a horizontal junction, wherein the first n-doped region is on a first lateral side of the modulator and the second n-doped region is on a second lateral side of the modulator.
10. The MOSCAP modulator of any preceding claim, wherein the modulator has an operating bandwidth in the range 30 GHz to 40 GHz.
11. The MOSCAP modulator of any preceding claim, wherein the first n-doped region, the second n-doped region and the p-doped region are formed from the same semiconductor material. 12. The MOSCAP modulator of any one of claims 1 to 10, wherein the first n-doped region is formed of a different semiconductor material than the second n-doped region and the p-doped region.
13. The MOSCAP modulator of any one of the preceding claims, wherein at least one of the first n-doped region, the second n-doped region, and the p-doped region is formed of a III-V semiconductor.
14. The MOSCAP modulator of claim 13, wherein the III-V semiconductor is indium phosphide.
15. A method for fabricating a MOSCAP modulator, the method comprising, on a substrate: growing a first semiconductor region and doping it with an n-type dopant to form a first n-doped region; growing an insulator on a first surface of the first n-doped region; growing a second semiconductor region on a second surface of the insulator, the first surface being opposite the second surface; doping a first portion of the second semiconductor region with a p-type dopant to form a p-doped region adjacent to the insulator; and doping a second portion of the second semiconductor region with an n-type dopant to form a second n-doped region adjacent to the p-doped region, wherein the insulator region is grown to extend at an oblique angle through a modulation region, the modulation region comprising an n-i-p-n junction formed from the first semiconductor region, the insulator, the first portion of the second semiconductor region, and the second portion of the second semiconductor region, wherein an interface between the insulator region and the first n-doped region is oblique, and an interface between the insulator region and the p-doped region is oblique, and wherein the first n-doped region and the second n-doped region are spaced apart in both horizontal and vertical directions.